With the advent of an aging society, there is a growing number of hearing-impaired elderly people. Many of these hearing-impaired elderly people suffer from presbyacusis involved in the aging process. Most of the presbyacusis is so-called sensorineural hearing loss, which is caused by a defect in the inner ear or in the nervous system connected to the inner ear. In other words, the presbyacusis is due to impaired propagation of sound signals caused by weakening, deformation, depletion or such of hair cells in the inner ear, which are supposed to convert the sound signals into signals that are transmitted to the brain, or caused by damage to the nerve that transmit the converted signals to the brain, with aging.
Conventionally, hearing aids have been provided as hearing assistance for hearing-impaired persons with lower-than-normal hearing. The hearing aids use a hearing aid technique that improves hearing by amplifying sound according to an extent of impairment of hearing characteristics of a hearing-impaired person, for example. Recently, speech-rate conversion has also been proposed as a hearing aid technique for improving hearing of words for the elderly, and thus there has appeared not only hearing aids but also a large number of televisions, radios, telephones, etc., with a function of reproducing speech slowly.
However, these hearing-aid appliances using the hearing aid technique merely improve part of mechanisms of hearing impairment. This means that the hearing aids which only amplify sound according to the hearing characteristics will not produce sufficient effects of hearing improvement for hearing-impaired persons with the sensorineural hearing loss including the presbyacusis. This is because the sensorineural hearing loss is not a state where it is difficult to hear simply in terms of sound volume, but is rather characterized by diminished ability for recognizing speech as words.
The characteristic ability impairment due to the sensorineural hearing loss includes 1) Loudness recruitment phenomenon, 2) reduced frequency selectivity, and 3) reduced temporal resolution, which are described in the following.
1) Loudness recruitment phenomenon indicates a phenomenon that a hearing-impaired person has an enhanced minimum audible level than a normal hearing listener, but for the hearing-impaired person, the loudness, which is a sound sensuous volume, rapidly grows when the sound intensity exceeds an audible level. That is, a hearing-impaired person with sensorineural hearing loss tends to be sensitive to changes in sound volume, having difficulty hearing low sounds but feeling sounds even a little higher than the audible level noisy. The above-mentioned conventional hearing aids using the hearing aid technique are intended to improve hearing by focusing on this phenomenon.2) In the case of the sensorineural hearing loss, the reduced frequency selectivity increases influences of masking of components in different frequency ranges, especially masking of high frequency components by low frequency components (so-called upward spread of masking). That is, hearing-impaired persons with sensorineural hearing loss tend to have more difficulty hearing sounds in the high tone range than sounds in the low tone range. In this regard, some disclosures indicate that separate input of low tones and high tones to right and left ears improves speech intelligibility (refer to Non-Patent Literature 1, for example).3) In the case of the sensorineural hearing loss, the reduced temporal resolution makes it difficult to respond to rapid sound changes. This therefore increases influences of temporal masking that one sound is masked by the other sound when two sounds are successively given, for example. That is, a hearing-impaired person with sensorineural hearing loss has difficulty in perceiving rapidly-changing sounds or in distinguishing temporally-close sounds. The temporal masking includes two types: forward masking, in which a preceding sound masks the following sound, and backward masking, in which a preceding sound is masked by the following sound. The forward masking indicates a phenomenon that when a person responds to a certain sound, the response to that sound will not be settled down soon after the loss of the sound, with the result that the following sound generated during the period becomes hard to hear. The backward masking indicates a phenomenon that because the neural response is quicker to louder sounds, a loud sound coming after a soft sound makes these two sounds indistinguishable from each other, with the result that the preceding soft sound becomes hard to hear.
In an ordinary conversation, vowels are characterized by high energy, small temporal changes, and long duration, while consonants are characterized by low energy, rapid changes, and short duration. Accordingly, although depending on a speaking speed in a conversation, a hearing-impaired person with sensorineural hearing loss often finds it difficult to hear consonants because they are prone to temporal masking by vowels before and after them.
Furthermore, a hearing-impaired person with sensorineural hearing loss who has difficulty responding to rapid sound changes because of reduced temporal resolution often misses a consonant even with no temporal masking by sounds before and after the consonant. This is because consonants, which rapidly change with short duration, are lost before hair cells of the hearing-impaired person with sensorineural hearing loss respond, and the hearing-impaired person is therefore not able to respond to such consonants. As a result, the hearing-impaired person misses the consonants.
As above, hearing-impaired persons with sensorineural hearing loss find it difficult to hear consonants because of the reduced temporal resolution and therefore are unable to know what is told or hear wrong, which decreases the consonant recognition ratio.
To deal with this, there is conventionally a method of reducing influences of the temporal masking. For example, there is a disclosed technique that, in order to prevent a vowel from temporally masking a consonant, signals of the vowel in low-frequency band with high formant components are suppressed, thereby emphasizing the consonant (refer to Patent Literature 1, for example). Another disclosed technique is that between a vowel and a consonant, a soundless segment is provided by suppressing part of a tail part of the vowel for a specific time, thereby reducing influences of temporal masking on an incoming consonant (refer to Patent Literatures 2 and 3, for example). There is still another proposed technique that provides right and left ears with respective signals having different frequency characteristics in order to reduce masking which relates to the temporal masking of a consonant by a vowel and occurs between frequency components (refer to Patent Literature 4, for example).
These processing can reduce the temporal masking of a consonant by a vowel and thereby improve hearing of consonants.